This invention relates to a beam quality detection system and more particularly to a system utilizing a single image detector.
Image motion detectors capable of tracking the trajectory of a plane or other moving objects, utilizing radiation enamating therefrom, are known in the art. Typically, radiation emanating from a moving body is continuously focused onto a rotating chopper or scanning disk which includes alternate zones of different transparency to the incident rays which modulate the intensity of the rays passing through the rotating disk according to the coordinate position of the alternate zones in the image field. The modulated intensity is detected by optical means which generate amplitude and frequency information proportional to the coordinates of the image point on the disk.
Schmutz in U.S. Pat. No. 3,307,038 discloses a rotating chopper disk adapted for periodically interrupting the passage of radiation from a search object to a detector. The rotating disk has an outer circular ring-shaped track having the form of a radial gap pattern which is periodic in the circular direction of the track movement and which repeats itself in sequential sector-forming sections. A dividing line extending diagonally across the sector-forming sections divides the sector-forming section into two fields wherein the radial gap pattern in each field is different. Motion of the image across the dividing line results in a frequency variation in an output signal from the detector which is analyzed by means well known in the art to determine the direction of the motion of the search object.
Astheimer in U.S. Pat. No. 3,090,869 discloses a motion detector device which utilizes a reticle in the form of a drum or a disk. The reticle includes bands of opaque bars and clear bars interspersed with one another wherein the bands are disposed at an angle of 45.degree. to the reticle travel direction and at right angles to one another. On the outer edge of the reticle are provided two sets of phase reference patterns for elevation phase and azimuth phase references. The error signal is developed with respect to the two orthogonally disposed bands which produce frequency and phase variations proportional to the radial and azimuthal motion of the image of the object being tracked.
Malueg in U.S. Pat. No. 3,950,099 discloses a two-axis image motion detector adapted for detecting two separate velocity channels through a single optical channel with a single photodetector. A single disk rotates at a substantially constant rotational speed and contains a pair of orthogonal grid patterns of different spatial frequencies. In the preferred embodiment each grid pattern is disposed at a 45.degree. angle with respect to a radial axis extending from the center of the disk through the grid pattern. An image focused on the grid pattern modulates the carrier frequencies determined by the grid patterns thereby generating a composite output in a single photodetector representing the sum of the two image motion channels. Standard filtering and demodulating techniques are utilized to generate a DC voltage proportional to the image rate for each of the orthogonal axes.
Prior art devices are adapted for sensing the radial and azimuthal motions of an object but are not adapted for determining the optical quality of a beam of radiation. In particular, beam astigmatism and defocus measurements are not possible and the prior art devices are not adapted for providing error signals to a feedback circuit capable of minimizing the degradation of the optical quality of a beam due to beam jitter, astigmatism or defocus.